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goes to the BPCU
* GCU sends a close command to the related GCB after the
BPCU sees all applicable breakers open.
APU Power
These components control APU power:
* APU GCU (AGCU)
* BPCU
* Engine GCUs.
AC GENERATION - BREAKER CONTROL - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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The AGCU monitors the power quality at the starter-generator
and power feeder lines in power distribution panel 1. The AGCU
uses a neutral current transformer (NCT) and differential
protection current transformer (DPCT) to monitor current
related properties.
The AGCU signals the BPCU to remove any other power source
off the related transfer bus before the APU power comes on to a
transfer bus.
Selection of APU generator switch 1 temporarily to the ON
position causes this sequence:
* AGCU monitors power quality of the APU starter- generator
(the sequence only continues if APU power is good)
* AGCU signals the BPCU of APU power selection
* BPCU signals GCU1 to open GCB1 if closed
* BPCU signals the EPC to open if closed
* AGCU signals the APB to close
* BPCU signals GCU1 to close BTB1
* If no power is on transfer bus 2, the BPCU signals GCU2 to
close BTB2.
You must put APU generator switch 2 temporarily to the ON
position if transfer bus 2 has power from another source.
Engine Generator Power
The GCU monitors the power quality at the IDG and at the
feeder lines in the power distribution panels. The GCU uses
differential protection current transformers (DPCT) to monitor
current related properties.
The GCB connects the IDG (generator) to the transfer bus.
Before an engine generator comes on to the transfer bus, the
GCU signals the BPCU to remove any other power source off
the related transfer bus.
Selection of engine generator switch 1 causes this sequence:
* GCU commands BTB1 open if other power sources are on
* BTB1 status goes to the BPCU
* BPCU sends an open command to the APB if it only powers
the related transfer bus
* BPCU sends an open command to the EPC if it only powers
the related transfer bus
* If the power is good, the GCU sends a close command to the
GCB
* BTB1 and BTB2 close if no other power source is on the
airplane.
A similar sequence occurs for the engine generator switch 2. If
you close both generator switches, both BTBs open.
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EFFECTIVITY
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24-20-00-150
Automatic Function
Usually, you must select the electrical power source to use it.
However, both GCBs automatically close and the IDGs supply
power if these conditions happen:
* APU supplies power to both transfer buses
* Airplane leaves the ground
* APB opens
* IDG power quality is good.
In this type of situation, these are the reasons why the APB may
open (trip):
* Crew shuts down APU
* APU starter-generator current overload
* APU starter generator power quality problem.
Bus Transfer Switch
The BPCU monitors bus transfer switch position for BTB
control. The BTB operation is automatic when the bus transfer
switch is in the AUTO position. The AUTO position is the normal
position of the switch.
You control the BTBs when you put the switch to the OFF
position. The BTB control you have depends on the electrical
power source conditions and air ground logic before you move
the switch to the OFF position. Here are the examples of what
happens when you put the bus transfer switch to the OFF
position:
* Lock both BTBs open (can not close) if each AC transfer bus
has power from its respective IDG
* Open the respective BTB of an IDG that supplies power to
both AC transfer buses (stops bus transfer to opposite AC
transfer bus)
* If one AC transfer bus has power from its IDG, and the
opposite AC transfer bus has power from the APU or
external power, the BTB related to the IDG opens and locks
open
* On ground, no change of BTB position happens if both AC
transfer buses have power from the APU starter-generator
* On ground, no change of BTB position happens if both AC
transfer buses have power from the external power source.
AC GENERATION - BREAKER CONTROL - FUNCTIONAL DESCRIPTION
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